Waveform generating circuit and spread spectrum clock generator

ABSTRACT

A spread spectrum clock generator is provided which improves the spread spectrum effect with little increasing the circuit cost by modifying the shape of a triangular wave used for frequency modulation by a simple method. The output signal of the modulation waveform generating circuit has such a modulation waveform as indicated by solid lines in FIG.  2 A. The modulation waveform is input to a VCO (voltage-controlled oscillator). In response to the modulation waveform, the oscillation frequency of the VCO is modulated, and the output clock that varies its frequency as illustrated in FIG.  2 B is obtained. The frequency transition of the output clock involves such temporal variations as indicated by solid lines in FIG.  2 C.

This application claims priority from Japanese Patent Application No.2004-363094 filed Dec. 15, 2004, which is incorporated hereinto byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a waveform generating circuit formaking certain modifications to a triangular wave signal, and to aspread spectrum clock generator using the waveform generating circuit.

The spread spectrum clock generator refers to a clock generator forreducing electromagnetic radiation by varying the clock frequency by theso-called spread spectrum clock technique.

2. Description of the Related Art

In the field of the digital circuit, it is known to vary the clockfrequency slightly to reduce the electromagnetic radiation caused by theclock.

To obtain the frequency modulated clock, it is generally performed toinput a triangular wave to a VCO (voltage-controlled oscillator).

FIG. 1 shows an example of a triangular wave generating circuitconventionally known. The input signal to the circuit is a rectangularwave, and complementary switch 22 and switch 23 which are driven throughan inverter 20 repeat turning on and off alternately. Thus, a capacitor(CAP) 25 is supplied with the current from a current source 21 and thecurrent from a source 24 alternately. In this way, the CAP 25 repeatscharge and discharge, thereby outputting a triangular wave.

Japanese patent application laid-open No. 06-242851 discloses thefollowing object and configuration (in abstract and FIG. 1). Its objectis “to make the interference visually inconspicuous on a screen causedby spurious radiation being fed to a TV tuner, which is generated bydigital circuits such as a microcomputer and the like”. Itsconfiguration is “a circuit for generating a clock pulse of amicrocomputer, which includes pulse generating means for generating apulse signal with a fixed frequency; and random modulation means formodulating the pulse signal at random the pulse generating meansgenerates, wherein the random modulation means includes a triangularwave generator; a frequency modulator for carrying out frequencymodulation of the triangular wave produced by the triangular wavegenerator; and a frequency converter for carrying out frequencyconversion based on the output of the frequency modulator and a signalobtained from the pulse generator, and wherein the output of thefrequency converting means is used as the clock pulse”.

However, when the frequency characteristic applied for the input to themodulation circuit or the oscillator is insufficient for achieving thefrequency modulation by the triangular wave as described above, therearises a problem in that the vertices of the triangular wave round, andideal spread spectrum is not achieved.

When the spread spectrum is brought about by the triangular wave withsuch a shape, a spectrum is observed which has higher level portions atboth ends of the spread spectrum than in other regions, which presents aproblem in that the effect of the spread spectrum is impaired.

On the other hand, even if the frequency modulation is carried out usinga triangular wave with an ideal shape, since the EMI (electromagneticinterference) measurement is made based on the finite frequencyresolution and measurement integral time, the frequency transition staysat frequencies near the vertices of the triangular wave longer than inother frequency ranges to be measured. As a result, a problem arises inthat the spectrum is observed which has the higher level portions thanin the other regions (see FIG. 7A).

To circumvent such a problem, a technique is conceived which carries outmodulation after changing the waveform for the frequency modulation to aparticular shape. The technique, however, must pay extra cost in both acircuit area and cost (see Electromagnetic Compatibility EMC, May, pp.22-28).

In view of the foregoing problems, a first object of the presentinvention is to provide a waveform generating circuit for generating amodified triangular wave signal suitable for being input to a frequencymodulation circuit such as a VCO (voltage-controlled oscillator).

A second object of the present invention is to provide a spread spectrumclock generator capable of improving the spread spectrum effect withlittle increase in the circuit cost by modifying the shape of thetriangular wave used for the frequency modulation with a simple method.

SUMMARY OF THE INVENTION

To accomplish the object, according to one aspect of the presentinvention, there is provided a waveform generating circuit comprising:triangular wave generating means for generating a triangular wave signalhaving first slope sections and second slope sections; offset generatingmeans for generating first offset component signals corresponding to thefirst slope sections, and for generating second offset component signalscorresponding to the second slope sections, the second offset componentsignals differing from the first offset component signals; combiningmeans for adding the triangular wave signal generated by the triangularwave generating means and the offset component signals generated by theoffset generating means; and output means for delivering an outputsignal resulting from the addition by the combining means.

The triangular wave generating means can include: a fixed capacitor; andcharge and discharge means for causing the fixed capacitor to be chargedand discharged via a pair of switches turning on and off complementarilyin response to an input clock signal, wherein a voltage corresponding tothe first slope sections and the second slope sections is obtainedacross the fixed capacitor.

The offset generating means can be a fixed resistor for generating asits terminal voltage the first offset component signals and the secondoffset component signals by causing a charging current and a dischargingcurrent of the fixed capacitor to flow through the fixed resistor; thecombining means can be a wire connection that connects the fixedcapacitor and the fixed resistor in series; and the output means can bea terminal to which the fixed resistor is not connected among the twoterminals of the fixed capacitor.

According to another aspect of the present invention, the offsetgenerating means can be an auxiliary capacitor; the combining means canbe a wire connection that connects the auxiliary capacitor across theinput terminal of the input clock signal and a terminal of the fixedcapacitor; and the output means can be a common connection point of theauxiliary capacitor and the fixed capacitor.

According to still another aspect of the present invention, the firstoffset component signals and the second offset component signals canhave opposite polarities to each other.

The waveform generating circuit can generate an asymmetric wave havingan operation level that varies at a predetermined period, and that isasymmetric before and after a maximum point and a minimum point of thewaveform.

The asymmetric wave can have a first slope section that makes atransition from a first low level to a first high level in apredetermined time period; a first level transition that makes aninstant transition from the first high level to a second high level; asecond slope section that makes a transition from the second high levelto a second low level in a predetermined time period; and a second leveltransition that makes an instant transition from the second low level tothe first low level.

The first slope section, the first level transition, the second slopesection, and the second level transition are generated repeatedly inthis order at a predetermined period.

According to another aspect of the present invention, there is provideda waveform generating circuit including: means for outputting data on awaveform of an asymmetric wave having an operation level that varies ata predetermined period, and that is asymmetric before and after amaximum point and a minimum point of the waveform; and a DAC forconverting the data on the waveform to an analog signal, and fordelivering an output signal.

The means for outputting can be one of a microprocessor, DSP, ROM and aprogrammable memory.

According to another aspect of the present invention, there is provideda spread spectrum clock generator including: the waveform generatingcircuit; and a variable frequency clock generating circuit that receivesthe output signal delivered by the waveform generating circuit, andgenerates a clock signal with a frequency corresponding to the outputsignal.

The means for outputting can be one of a microprocessor, DSP, ROM and aprogrammable memory.

The present invention can implement a waveform generating circuit forgenerating a modified triangular wave signal suitable for inputting to afrequency modulation circuit such as a VCO. In addition, according tothe present invention, the shape of the triangular wave used for thefrequency modulation can be modified by a simple method, which makes itpossible to implement the spread spectrum clock generator capable ofimproving the spread spectrum effect without increasing the circuit costso much.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing an example of a conventionaltriangular wave generating circuit;

FIGS. 2A-2C are diagrams showing a spread spectrum clock generator inaccordance with the present invention, and signal waveforms of variousportions;

FIG. 3 is a diagram showing a method of generating a modulation waveformrepresented by solid lines in FIG. 2A;

FIG. 4 is a circuit diagram showing a concrete configuration of themodulation waveform generating circuit 10 as shown in FIG. 2A;

FIG. 5 is a circuit diagram showing another concrete configuration ofthe modulation waveform generating circuit 10 as shown in FIG. 2A;

FIGS. 6A-6C are diagrams illustrating comparison between the effect ofthe spread spectrum by the conventional modulation wave generatingcircuit (FIG. 1) and the effect of the spread spectrum by embodiments(FIGS. 2A-5) in accordance with the present invention;

FIGS. 7A and 7B are diagrams illustrating in more detail a reductioneffect or flattening of the spectrum as shown in FIG. 6C;

FIG. 8 is a circuit diagram showing another concrete configuration ofthe modulation waveform generating circuit 10 as shown in FIG. 2A;

FIG. 9 is a circuit diagram showing still another concrete configurationof the modulation waveform generating circuit 10 as shown in FIG. 2A;

FIG. 10A is a circuit diagram showing a concrete configuration of theVCO 11, and FIG. 10B is a diagram illustrating relationships between anordinary input voltage and an output frequency of the VCO 11;

FIG. 11 is a diagram illustrating a method of generating the modulationwaveform represented by solid lines in FIG. 2A; and

FIG. 12 is a diagram illustrating another method of generating themodulation waveform represented by solid lines in FIG. 2A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 2A-2C show a spread spectrum clock generator of an embodiment inaccordance with the present invention, and signal waveforms of variousportions. In these figures, the reference numeral 10 designates amodulation waveform generating circuit whose output signal has amodulation waveform as illustrated by solid lines in FIG. 2A. Themodulation waveform is input to a VCO (voltage-controlled oscillator)11. Thus, the oscillation frequency of the VCO 11 is modulated inresponse to the modulation waveform, providing the output clock withfrequency variations as illustrated in FIG. 2B. The frequency transitionof the output clock has temporal variations as indicated by solid linesof FIG. 2C.

FIG. 3 shows an example of a method of generating the modulationwaveform represented by solid lines in FIG. 2A. FIG. 3( a) shows a puretriangular wave, and FIG. 3( b) shows a rectangular wave for an offset,which has transition points at the same timing as the vertices of thetriangular wave. Adding the triangular wave of FIG. 3( a) and therectangular wave of FIG. 3( b) results in the waveform as illustrated inFIG. 3( c), which shifts sides from vertices to the next vertices of thetriangular wave upward or downward.

FIG. 11 illustrates another method of generating the modulation waveformindicated by the solid lines in FIG. 2A. FIG. 11( a) shows a puretriangular wave as shown in FIG. 3( a). FIG. 11( b) shows a rectangularwave for an offset, which has transition points at the same timing asthe vertices of the triangular wave. Although the rectangular wave ofFIG. 3( b) has a fixed signal level with its polarization reversed atthe transition points, the rectangular wave of FIG. 11( b) has itssignal level changed at the transition points without the polarizationreversal. Accordingly, adding the triangular wave of FIG. 11( a) and therectangular wave of FIG. 11( b) results in the waveform as illustratedin FIG. 11( c), which shifts sides from vertices to the next vertices ofthe triangular wave upward or downward in accordance with the signallevels.

FIG. 12 illustrates still another method of generating the modulationwaveform indicated by the solid lines in FIG. 2A. FIG. 12( a) shows apure triangular wave as FIG. 3( a). FIG. 11( b) shows a rectangular wavefor an offset, which has the transition points at the same timing as thevertices of the triangular wave, but has the opposite polarity to thatof FIG. 3( b). Specifically, the rectangular wave in FIG. 12( b) has thenegative polarity in regions in which the rectangular wave in FIG. 3( b)has the positive polarity, and has the positive polarity in regions inwhich the rectangular wave in FIG. 3( b) has the negative polarity.Thus, adding the triangular wave of FIG. 12( a) and the rectangular waveof FIG. 12( b) results in the waveform which slopes up toward the right,has a step at the top, slopes down toward the right and has a step atthe bottom.

FIG. 4 shows a concrete circuit configuration of the modulation waveformgenerating circuit 10 as shown in FIG. 2A. The input to the circuit is arectangular wave as in the conventional example of FIG. 1, andcomplementary switch 42 and switch 43 which are driven through aninverter 40 repeat turning on and off alternately. Thus, a capacitor(CAP) 45 is supplied with the current from a current source 41 and thecurrent from a current source 44 alternately. As a result, the CAP 45repeats charge and discharge. When the charge and discharge arerepeated, an offset voltage is generated across a resistor 46 connectedbetween the CAP 45 and a ground. Thus, while the switch 42 is in the ONstate and the switch 43 is in the OFF state, the offset voltage, whichis generated with an amount of (resistance of the resistor 46)×(supplycurrent of the current source 41), is added to the output. On the otherhand, while the switch 42 is in the OFF state and the switch 43 is inthe ON state, the offset voltage, which is generated with an amount of(resistance of the resistor 46)×(supply current of the current source44), is subtracted from the output. The addition and subtraction canprovide as the output waveform the waveform that shifts the sides fromvertices to the next vertices of the triangular wave upward or downward.

FIG. 5 shows another concrete circuit configuration of the modulationwaveform generating circuit 10. The input to the circuit is also arectangular wave, and repeating alternate turning on and off of thecomplementary switches 32 and 33 driven by the inverter 30 causes thecurrents from the current source 31 and current source 34 to flow into acapacitor (CAP) 35. As a result, the charge and discharge of the CAP 35are repeated. In addition, a capacitor (CAP) 36 is connected across aninput terminal and an output terminal in the present circuit.Accordingly, as long as the input rectangular wave is at a high level,the extra charges determined by the capacitance of the CAP 36 and by thecapacitance of the CAP 35 are stored in the CAP 35. As a result, as longas the input rectangular wave is at the high level, the upward slopevoltage across the CAP 35 is shifted upward. On the other hand, as longas the input rectangular waveform is at a low level (zero volt), thecharge of the CAP 35 flows into the CAP 36. Thus, the downward slopevoltage across the CAP 35 is shifted downward.

FIG. 8 shows another concrete circuit configuration of the modulationwaveform generating circuit 10. The input terminal is connected to theinverter 30, and the output terminal of the inverter 30 is divided intothree branches. Two of the three branches are connected to the switches32 and 33 to turn on and off the switches in response to the high andlow of the signal output from the inverter 30. The current sources 31and 34 and the switches 32 and 33 are connected in series in the orderof the current source 31, switch 32, switch 33 and current source 34.The current source 34 has its end, which is not connected to the switch33, connected to the ground. The output terminal is connected to theconnecting point of the switch 32 and switch 33. The CAP 35 is connectedacross the output terminal and the ground. The CAP 36 is connectedacross the output terminal and the output of the inverter 30.

Although the CAP 36 of the modulation waveform generating circuit shownin FIG. 5 is connected across the input terminal and the outputterminal, the CAP 36 of the modulation waveform generating circuit ofFIG. 8 is connected across the output of the inverter 30 and the outputterminal. Changing the connecting position of the CAP 36 can reverse thedirections of the steps of the output waveform because the CAP 36 issupplied with the voltage opposite in polarity to the voltage suppliedto the CAP 36 shown in FIG. 5.

The directions of the steps of the output waveform can also be changedby reversing the polarity of the switches 32 and 33. Specifically, thedirections of the steps of the output waveform can be altered bychanging whether the switches are closed when the output of the inverter30 is high or low.

FIG. 9 shows another concrete circuit configuration of the modulationwaveform generating circuit 10. The modulation waveform generatingcircuit 10 has a ROM 91 and a digital-to-analog converter (DAC) 92. TheROM 91 stores data on the waveform in advance which has offsets in theslopes. The ROM 91 supplies its output data string to the DAC 92, andthe DAC 92 converts the data to an analog signal and outputs it as theoutput signal. Thus, the waveform having the offsets in the slopes canbe produced as in the foregoing embodiment. Instead of the ROM 91, it isalso possible for the present embodiment to use a processing unit suchas a programmable memory, microprocessor or DSP (digital signalprocessor) (not shown) to generate a prescribed data string, therebybeing able to produce the waveform having the offsets in the slopes justas in the case of using the ROM 91.

FIG. 10A shows a concrete circuit configuration of the VCO 11. The VOC11 includes inverters 104-106 whose speed can be varied by a current,current sources 101-103 for supplying currents to the inverters 104-106,and a V/I (voltage/current) converter 100 for determining the currentsto be supplied to the inverters 104-106 by controlling the currentsources 101-103 in response to the voltage. The voltage supplied via theinput terminal is converted to the current corresponding to the voltageby the V/I converter 100, and the current is supplied to the inverters104-106. FIG. 10B illustrates relationships between the ordinary inputvoltage to the VCO 11 and the output frequency. Generally, the frequencyof the output signal increases with the current supplied to theinverters 104-106. Thus, the output frequency increases with the inputvoltage. However, it is also possible to decrease the output frequencywith the input voltage by changing the polarity of the V/I converter100.

FIGS. 6A and 6B are graphs illustrating for comparison the effects ofthe spread spectrum by the conventional modulation wave generatingcircuit (see FIG. 1) and the spread spectrum by the embodiments inaccordance with the present invention (see FIG. 2A to FIG. 5). FIG. 6Ashows a spectrum when a sine wave or rounded triangular wave is used asthe frequency modulation waveform. Since the frequency modulationbecomes mild near the top and bottom vertices of the sine wave, thespread spectrum has a shape with peaks at both ends FIG. 6B shows aspectrum when a triangular wave is used as the frequency modulationwaveform. When the triangular wave is used as the frequency modulationwaveform, although the temporal change of the frequency modulation isconstant, the EMI measurement is made at a finite frequency resolution.Accordingly, the frequency transition stays in the frequency resolutionat the vertices of the triangular wave for a time longer than othertimes, resulting in the spread spectrum having peaks at both ends aswell. In contrast with the two examples, the embodiments in accordancewith the present invention employ the waveform as shown in FIG. 6C,which shifts the sides from the vertices to the next vertices of thetriangular wave upward or downward. Thus, considering the vertices ofthe waveform, the stay duration at a fixed frequency band becomesconstant. As a result, the peaks at both ends of the spectrum can bereduced or flattened.

FIGS. 7A and 7B are diagrams illustrating the reduction effect orflattening of the spectrum shown in FIG. 6C in more detail. FIG. 7A is adiagram illustrating the case where the modulation waveform is atriangular wave as in the conventional example, and the frequencyresolution of the EMI measurement is Δf. As is clear from FIG. 7A,assuming that the time passing through the frequency region Δf is Δt inportions other than the vertices of the triangular wave, then the timepassing through the frequency region Δf at the vertices is 2Δt includingthe time of a turn. Thus, in this frequency region, the spectrum isobserved which is higher than in the other portions. On the other hand,FIG. 7B is a diagram illustrating the embodiments in accordance with thepresent invention, which employs the waveform that shifts the sides fromthe vertices to the next vertices of the triangular wave upward ordownward as the modulation waveform. As is clear from FIG. 7B, as forthe waveform of the present embodiment, the time for passing through thefrequency region with the frequency resolution Δf is identical for theportions at the vertices and for the other portions, the two ends of thespread spectrum can be flattened.

The present invention has been described in detail with respect topreferred embodiments, and it will now be apparent from the foregoing tothose skilled in the art that changes and modifications may be madewithout departing from the invention in its broader aspect, and it isthe intention, therefore, in the apparent claims to cover all suchchanges and modifications as fall within the true spirit of theinvention.

1-5. (canceled)
 6. A waveform generating circuit characterized bygenerating an asymmetric wave having an operation level that varies at apredetermined period, and that is asymmetric before and after a maximumpoint and a minimum point of the waveform.
 7. The waveform generatingcircuit as claimed in claim 6, wherein the asymmetric wave has a firstslope section that makes a transition from a first low level to a firsthigh level in a predetermined time period; a first level transition thatmakes an instant transition from the first high level to a second highlevel; a second slope section that makes a transition from the secondhigh level to a second low level in a predetermined time period; and asecond level transition that makes an instant transition from the secondlow level to the first low level.
 8. The waveform generating circuit asclaimed in claim 7, wherein the first slope section, the first leveltransition, the second slope section, and the second level transitionare generated repeatedly in this order at a predetermined period.
 9. Awaveform generating circuit as claimed in claim 6 further comprising:means for outputting data on a waveform of the symmetric wave having anoperation level that varies at a predetermined period, and that isasymmetric before and after a maximum point and a minimum point of thewaveform; and a DAC for converting the data on the waveform to an analogsignal, and for delivering an output signal.
 10. The waveform generatingcircuit as claimed in claim 9, wherein said means for outputting has oneof a microprocessor, DSP, ROM and a programmable memory.
 11. A spreadspectrum clock generator comprising: a waveform generating circuit asclaimed in one of claims 6-10; and a variable frequency clock generatingcircuit that receives the output signal delivered by said waveformgenerating circuit, and generates a clock signal with a frequencycorresponding to the output signal.